There are 300 million people in the US. Then there's 30M from canada, and let's be generous and say 30M more.
Now, the average is ~ 3.5 people pr family. Let's be generous and say 3 people.
That means there are, currently, 120M homes.
Now, over the next 5 years (which the Wii are going to sell during), some of these homes will expand (due to kids moving out, making a new home. The Wii stays at the original house, and they buy a new one for the next), so let's be a bit generous and say 150M.
So there are, maximum, 150M homes in the Americas (140M is more likely).
I can't think of anything, in any country, that has a 66% penetration rate (things that are made by one company), can you?
Now, 100M just starts seeming even more hopeless, considering that the Ps3 and X360 exist as well.
Case is, they're going to sell close to 50M combined, but let's be generous (for the Wii), and say 40M (that's just 18M more...)
Now, let's say that 50% of those Ps360 owners also own a Wii (unlikely). That means that 20M of the 150M houses are gone to the Ps3/X360.
The Wii now needs a 75% penetration rate. I sincerely doubt it (or anything!) can make that.